The fabrication of printed circuit boards using deposition of conductive metals is well known in the art. Methods for depositing the conductive metals include vapor deposition, electrodeposition, electroless deposition, and sputtering. Once a uniform layer of metal is deposited onto a substrate, then the circuit is formed by subtractive imaging which etches the unwanted metal away from the metallized surface. In a subtractive imaging process, a conductive metal is first deposited over the entire surface of the substrate, a photoresist is then coated and dried on the metal surface, followed by imaging and development of the image. The exposed metal is etched away using a etching solution and the remaining photoresist is removed from the metallic image. This process is very time consuming and wasteful of materials. In addition, the disposal of the developing solutions and/or etching solutions used in the process creates environmental concerns.
Alternatively, the metal may be deposited as a positive image. The most common method for forming a positive image is screen printing using a metallic conductive ink. A typical metallic ink contains a conductive metal dispersed in a binder and a solvent. The positive image is formed by screen printing the metallic ink onto a substrate followed by removal of the solvent. When possible, the imaged substrate is then subjected to high temperatures to volatilize the residual binder. If the residual binder is not removed, then the binder may interfere with the conductivity of the metal image. Even though the process steps of a positive imaging method are fewer than those of the subtractive method, the screen printing method has limited resolution. In addition, the screen printing method requires the removal of solvents which is again an environmental concern.
There have been many attempts to improve the productivity of the process by using lasers to deposit the conductive metal onto a substrate. For example, U.S. Pat. No. 5,378,508 discloses a method for providing a conductive metal deposit on a substrate by applying a metallic salt composition onto a substrate and then imaging the composition using a coherent radiation source. The radiation reduces the metal salt in the presence of an amine or amide compound to form a metallic image. The imaged substrate is then washed and dried to remove the non-imaged areas, thus requiring disposal of the washing solvent. The choice of useful metallic salts are also limited due to potential contamination by the anion.
U.S. Pat. No. 4,710,253 describes a circuit board which is manufactured by using a heat actuable adhesive in combination with a conductive powder. The conductive powder is applied over the surface of the adhesive and imaged using a coherent light source. In the light struck areas, the adhesive becomes tacky and the conductive powder adheres to the tackified adhesive. The excess metallic powder is brushed away. The imaged substrate is then fired to remove the adhesive and to fuse the metal to the substrate. The metallic powder necessarily has to be in contact with the adhesive during the imaging process due to the loss of adhesive tack after the imaging process. Since the metallic powder must be in contact with the adhesive during the imaging process, the image may become distorted due to the interference of the laser beam by the metallic powder. In addition, the metallic powder must be applied over the entire surface of the adhesive leading to excessive waste of materials.
U.S. Pat. No. 4,636,403 describes a process for repairing defects in a metal pattern of a photomask by coating the surface of the photomask with a metal-organic layer and then exposing the layer with a laser beam in the area of the defect to adhere the metal-organic layer to the defect area. The unexposed metal-organic layer is removed with a solvent.
U.S. Pat. No. 4,578,155 describes a process using a laser to locally plate a metal onto a surface without the need for an electrical potential being applied to the polymeric workpiece. The workpiece is placed in an electroplating solution and the laser is directed through the solution onto the polymer surface causing plating in the light struck areas.
U.S. Pat. No. 4,526,807 describes a process for forming conductive metal lines using a coherent radiation source. A solution containing a reducible metal or metalloid compound in an oxidizable organic matrix is solvent coated onto a substrate. An imagewise pattern is then directly written onto the coated layer using a laser beam. The metal or metalloid compound is reduced to conductive metal in the light struck areas.
U.S. Pat. No. 3,451,813 describes a process is using a high intensity flash lamp to expose a photosensitive layer containing a reducible metal or metalloid compound. The photosensitive layer is applied to the substrate in a pattern and then exposed to form a conductive metal pattern. Even though this process does not require the use of etchants or wet processing chemicals, the resolution is limited by the methods used to apply a patterned photosensitive layer on the substrate.
U.S. Pat. No. 3,941,596 describes an imaging process that uses a radiation-sensitive or heat-sensitive layer which becomes tacky or fluid upon exposure to radiation. An image is formed by the application of a dry or liquid toner to the exposed areas of the layer. The conventional method for forming images with these materials is by using a thermal printhead. However, in U.S. patent application Ser. No. 08/319,934, a recording medium having a radiation-sensitive layer is described which is addressable by an infrared laser. U.S. Pat. No 5,286,604 also discloses a photothermotackifiable composition which is addressable by lasers. In each of these systems, the materials are used as a recording medium. There is no disclosure of their use as a means for providing a conductive metallic image on a printed circuit board.
The printed circuit board manufacturing methods to date suffer from either excessive process times, limitation of useful available materials, limited resolution, or waste of materials; therefore, there is a need for a more productive and efficient process of providing a conductive metallic image on a circuit board. In particular, there is a need for a method compatible with a laser addressable system to allow for automation and high resolution.